We plan to continue our long-term research involved in the characterization of specific steroid binding proteins for developing model systems to study, first the chemical nature of androgen, estrogen, progestin, and glucocorticoid binding sites, and second the role which these proteins play in the overall mechanism of action of steroid hormones. The chosen models are the sex steroid-binding plasma protein (SBP) of human serum which we have recently purified (Biochemistry 14, 957, 1975), Macaca nemestrina and baboon SBP, rabbit SBP, dog SBP, and human and rabbit CBG (corticosteroid-binding globulin). The experiments will involve chemical modification using both "group specific" reagents and affinity labels containing either photoreactive or haloacyl and diazonium groups. This approach will result in the identification of amino acid side chains in the binding site responsible for the specific adsorption of the steriod on the protein surface. The availability of these pure proteins will permit the preparation of specific antibodies to study the physiological role of SBP and CBG. These will be injected into rabbits, and androgen and cortisol responsive tissues will be examined morphologically. Dramatic histological changes and changes in steroid dynamics should result if the levels of "free" testosterone and cortisol increase as a result of systematic removal of plasma SBP and CBG caused by the antibody. In conclusion, the comparison of these various specific proteins will not only result in our further understanding of the chemical factors controlling specificity of steroid binding, but will also serve to establish animal models to study function.